The present invention relates to a unique word detection apparatus, and more particularly to an apparatus for detecting a unique word for use in a satellite communication system using a burst transmission.
In satellite communication, an earth station adds a preamble at the top of the data. The preamble comprises a carrier recovery section for recovering the carrier on the receiving side, a bit timing recovery section for recovering the clock on the receiving side, and a unique word which is distinguishable from other data. The preamble and the data modulate a carrier, and the modulated carrier is R/F converted and transmitted to the satellite as a burst.
Another earth station having received this burst via the satellite detects the unique word from the received burst, and extracts the data in the burst. Therefore, if this unique word fails to be detected or a bit pattern similar to the bit pattern of the unique word is mistaken for that of the unique word, the receiving earth station will be unable to correctly receive the transmitted data.
According to the prior art, the following method is used to detect a unique word out of a received burst. As the received burst, after being converted into an intermediate frequency band, is entered into a demodulator, a non-zero D.C. component will appear in a demodulated signal corresponding to the carrier recovery section of the received burst. The D.C. components of the demodulated signal corresponding to the bit timing recovery section, unique word and data section are virtually zero. According to the prior art, the rise timing of the D.C. component contained in the demodulated signal of the carrier recovery section, i.e. the start timing of the carrier recovery section, is detected. Since the timing of the unique word supply is predetermined in the burst in reference to the start timing of the carrier recovery section, the receiving side forms an aperture signal, on the basis of the detected start timing, to indicate the period during which the unique word is to be detected. A unique word detector detects the unique word from the demodulated signal only during the period designated by this aperture signal.
To prevent the aforementioned misdetection of the unique word, the unique word is detected only during the period designated by the aperture signal according to this prior art.
However, since the carrier phase acquisition is not fully established in the demodulator while the leading part of the carrier recovery section is being entered into the demodulator, the D.C. component in the demodulator's output does not take on a sufficiently large value. Therefore, even if it is attempted to detect the start timing of the carrier recovery section by using a suitable threshold, accurate detection of this start timing is impossible. According to the prior art, the failure to detect the unique word is prevented by providing a sufficient width for the aperture signal. This makes it impossible to fully prevent the aforementioned misdetection of the unique word, and thereby invites a deterioration of the reliability of the satellite communication network and an increase in the traffic volume due to retransmitted data.